The present invention relates to a process for manufacturing a composite flexible hose, particularly a composite flexible hose suitable for transporting cooling mediums.
Composite flexible hoses for transporting cooling gases for air conditioners of cars and for contact freezers should be not only flexible but also resistant to an external pressure and an internal pressure of gases transported therethrough. For example, JU-B-40392/1989 discloses a hose for transporting cooling mediums, having four-layer structure; an inner tube comprising a polyamide resin layer and an outer rubber layer, a reinforcement layer provided on the inner tube and an external rubber covering said reinforcement layer. For the polyamide resin layer of the hose, nylon 6, nylon 66, 6/66 copolymer nylon and mixed nylon thereof are used, and for the outer rubber layer, chlorosulfonated polyethylene (CSM), chlorinated polyethylene (CPE), and nitrile butadiene rubber (NBR) are used. Polyamide resin is excellent in heat resistance, fluorine impermeability, refrigerating machine oil (PAG or non-PAG) resistance and moisture impermeability.
For hoses for transporting cooling mediums, there is a case where negative pressure is applied to an inside wall thereof, and therefore adhesion among each layer has to be especially strong. In case of hoses of JU-B-40392/1989, adhesion between the polyamide resin layer and CSM and between CPE and NBR is insufficient, and occasionally negative pressure causes interface adhesive failure.
Meanwhile JP-A-110143/1989 discloses a composite flexible hose having a layer structure comprising, as shown in FIG. 4 in the order from the inside, an innermost layer 2, an adhesive layer 6, an intermediate rubber layer 3, a fibrous reinforcement layer 4 and an external rubber layer 5. In these hoses, a polyamide or a modified polyamide is mainly used as material for the innermost layer 2, while a butyl rubber composition is used for the intermediate rubber layer 3 in viewpoints of pliability, cold resistance and moisture impermeability. However, to bond those two layers, the adhesive layer 6 is provided between them, because adhesion of a polyamide to a butyl rubber is generally difficult. As an adhesive for the adhesion layer, mainly used is a solvent type phenol resin, a mixed solvent of a phenol resin and an elastomer, or an epoxy resin adhesive.
However, the use of the solvent type adhesive requires longer time for evaporation of solvent, and more than that, pollutes working environment. Also irrespective of solvent type, the provision of an adhesive layer causes increase in the number of process steps and cost. Therefore, it is originally desirable not to use any adhesive, but the situation is such that an adhesive layer has to be provided in order to obtain a strong adhesion between the layers.
Also U.S. Pat. No. 4,633,912 discloses a composite hose for Freon gas, comprising a polyamide core tube, an elastic friction layer having the specific composition and being directly provided on a core tube, a first reinforcement strand layer, an adhesive barrier friction layer, a second reinforcement strand layer, and then a cover layer. The elastic friction layer provided directly on the core tube comprises (a) a base rubber selected from EPDM, a copolymer of butadiene, polychloroprene, polybutadiene, polyisoprene or a mixture thereof, (b) a calcium ion source, (c) resorcinol or a phenol-based adhesive system, and (d) a peroxide or a sulfuric vulcanizing agent. The calcium source (b) is said to make better adhesion to a polyamide of the core tube. The adhesive barrier friction layer being present between the first and second reinforcement strand layers is provided to minimize a friction of the strands, and is made of a copolymer of ethylene and acrylic acid. For the cover layer, a halogenated butyl rubber containing bis-dienophile as a crosslinking agent is used.
Also various materials for hoses for transporting cooling mediums have been developed. For instance, Tokkyo Kohyo No. 507303/1993 (International Publication No. WO91/18050) discloses a hose composition being excellent in modulus of elasticity, tensile strength and elongation characteristic, and comprising a halogen-containing copolymer of isomonoolefin of C.sub.4 -C.sub.7 and para-alkylstyrene, which contains at least about 5% by weight of the para-alkylstyrene and at least about 0.4% by mole of halogen. The composition further comprises other blending components such as fillers and curing agents like brominated phenol resin. This hose composition is used as at least a part of materials for cooling solution transporting hoses such as an inner tube, a reinforcement material and an external cover. Such a hose is manufactured by extruding the hose composition in the form of tube to make the inner tube, forming the reinforcement layer with a braided fiber on the inner tube, extruding the same hose composition as the inner tube or the conventional material of the external cover onto the reinforcement layer, and then vulcanizing wholly.
Thus, very delicate consideration is required for the combination and selection of materials and layer structure for the inner tube and the rubber layer provided thereon, and the desired effect cannot be obtained only by optional selection of known materials.
For instance, the structure described in U.S. Pat. No. 4,633,912 in which butyl rubber and/or a halogenated butyl rubber having an excellent water impermeability are not used as a material for the intermediate rubber layer, may be provided with no adhesive layer. When butyl rubber and/or a halogenated butyl rubber are used to withstand strict operating conditions, however, an adhesive layer is necessary as described in JP-A-110143/1989.
The rubber composition for the elastic friction layer described in U.S. Pat. No. 4,633,912 and the hose composition described in Tokkyo Kohyo No. 507303/1993 are both inferior in adhesive strength to polyamides, particularly in adhesive strength after heat aging. Especially the former rubber composition has less adhesive strength under normal conditions, and particularly an adhesive strength after heat aging is extremely inferior (Refer to Blend Nos. 1-11 and 1-12 in Table 1).
An object of the present invention is to provide a composite flexible hose having an excellent adhesive strength between layers, even if there is no adhesive layer when butyl rubber is used for the intermediate rubber layer.
Another object of the present invention is to provide a process for manufacturing the composite flexible hose in which a process step for forming an adhesive layer can be eliminated, whereby time and cost can be saved.